The present invention relates to a rubber composition for use particularly in the manufacture of tire treads, having improved hysteretic and physical properties in the vulcanized state, and comprising a diene polymer functionalized with alkoxysilane and carbon black having silica fixed to its surface as reinforcing filler.
Since savings in fuel and the necessity of environmental protection have become a priority, it is desirable to produce polymers having good mechanical properties and the lowest possible hysteresis, in order to be able to use them in the form of rubber compositions usable in the production of various semi-finished products which enter into the composition of tire treads, such as e.g. underlayers, binding gums between rubbers of various types or tire treads or sheathings of metallic or textile reinforcements, sidewall rubbers or tire treads, and to obtain tires of improved properties, and particularly a reduced rolling resistance.
To attain this objective, numerous solutions have been proposed consisting, in particular, in modifying the nature of diene polymers and copolymers at the end of polymerization by means of coupling agents, starring agents or functionalizing agents. Essentially, the great majority of these solutions have concentrated on the use of polymers modified with carbon black as reinforcing filler, with a view of obtaining a good interaction between the modified polymer and carbon black, inasmuch as the use of a white filler, notably silica, has proved inappropriate owing to the low value of some properties of such compositions and hence of certain characteristics of tires using these compositions, such as low abrasion resistance. As illustrative prior-art examples, reference may be made to U.S. Pat. No. 4,677,165, which describes the reaction of living polymers functionalized with benzophenone derivatives to obtain improved properties of carbon black-containing compositions. With the same objective, U.S. Pat. No. 4,647,625 describes the functionalization of polymers by reacting living polymers with N-methylpyrrolidine. Patent Applications EP-A-0 590 491 and EP-A-0 593 049 describe polymers containing amine functions, which permit a better interaction between polymer and carbon black.
A few solutions have also been proposed concerning the use of silica as reinforcing filler in compositions intended for the production of tire treads. Thus, Patent Applications EP-A-0 299 074 and EP-A-0 447 066 describe functionalized polymers comprising alkoxysilane functions. Only these functionalized polymers have been described in the prior art as being effective in reducing hysteresis and improving the abrasion resistance; nevertheless, the properties of these polymers remain inadequate to permit their use in compositions intended for making tire treads. Moreover, the formulation of these polymers poses problems due the development of macrostructures during elimination of the polymerization solvent, which leads to severe deterioration of properties of potential interest. Furthermore, this development cannot be well controlled.
Moreover, coupling reactions are frequently observed during such functionalization reactions, and, in order to minimize them, an excess of alkoxysilane and/or intense mixing are generally used.
This interest in silica-containing compositions has recently been demonstrated by the publication of Patent Application EP-A-0 501 227, which discloses a rubber composition vulcanizable with sulfur, obtained by thermomechanical working of a conjugated diene copolymer and a vinylaromatic compound, and using a precipitated silica of particularly high dispersibility. This composition, which represents an excellent compromise between several contradictory properties, permits the manufacture of tires having a tread filled with a special silica.
However, it is necessary, in silica-based compositions, to use relatively large amounts of linking agents to ensure that said compositions retain a good mechanical strength. These materials have very low electric conductivities, which, under certain special circumstances, do not always permit efficient grounding of accumulated static electricity, and which make it desirable to use a more complex method of tire production, using old solutions that are taken up in U.S. Pat. No. 5,518,055, EP-A-0 705 722 or EP-A-0 718 126 which describe the use of a conductive rubber strip traversing the tire tread; or in Patent Application EP-A-0 556 890, which deals with the addition of special conductive polymers to the rubber composition which constitutes the tire tread.
More recently, Patent Applications WO 96/37547 and EP-A-0 711 805 have described rubber compositions using, as reinforcing filler, carbon black having silica fixed to its surface, and which are based on functionalized or nonfunctionalized diene polymer and a silane coating agent and coupling agent. These compositions seem to have hysteretic properties that are improved in comparison to compositions containing carbon black as reinforcing filler. However, the silane coating agent and coupling agent must always be used in relatively large quantities.
Furthermore, Patent Application EP-A-0 711 805 discloses that improved hysteretic properties are obtained with a composition having a functionalized polymer known to improve the hysteretic properties with carbon black, a coupling agent and carbon black having silica fixed to its surface as reinforcing filler, in comparison to a composition using silica of low dispersibility as reinforcing filler; however, this publication does not present any data on the variation of other properties, and on the physical properties in particular.